In conventional flat panel display systems, such as liquid crystal display (LCD) systems, the brightness of each pixel or element is controlled by a transistor. An active matrix display includes a grid of transistors (e.g., thin film transistors) arranged in rows and columns. A column line is coupled to a drain or a source associated with each transistor in each column. A row line is coupled to each gate associated with the transistors in each row. A row of transistors is activated by providing a gate control signal to the row line which turns on each transistor in the row. Each activated transistor in the row then receives an analog voltage value from its column line to cause it to emit a particular amount of light. Generally speaking, a column driver circuit provides the analog voltage to the column lines so that the appropriate amount of light is emitted by each pixel or element. The resolution of a display is related to the number of distinct brightness levels. For a high quality display, a multi-reference voltage generator (e.g., eight or more voltages) is needed to supply voltages to the column driver.
FIG. 1 shows an LCD display 102 along with portions of its driver circuitry, including column driver(s) 104, and a multi-reference voltage generator 106, which provides analog voltages to the column driver(s) 104. Although FIG. 1 shows the driver circuitry logically separate from the display 102, commercial displays may combine the display and the driver circuitry into a single thin package. Therefore, a major consideration in developing circuitry for such displays is the microchip die size required to implement the driver circuitry. Cost is also a factor to be taken into account.
To achieve multi-reference voltage outputs, digital-to-analog converters (DACs) can be used to generate different voltages. Capacitors can be coupled to the DACs to temporarily buffer the voltages. Such a multi-reference voltage circuit has been conventionally implemented in several ways. One way uses a multi-DAC structure as shown in FIG. 2, discussed below, wherein a separate DAC is used to drive a buffer for each of the N output channels. DAC circuits are very large, however. Accordingly, with such a multi-DAC structure, as the number of output channels increase, the chip die size will become undesirably large. What is needed is a multi-reference voltage buffer small enough to be used in flat panel display packages.
In TFT-LCD applications, column drivers drive storage capacitors in TFT-LCD cells. In large panel applications, such as in television and other monitor applications, the color accuracy of the LCD display becomes more important, as it is easily perceived by the human eye. Any mismatch between the capacitor cell voltages in the LCD cell could cause these color mismatches. The multi-reference voltage generator 106 is used to improve the accuracy and reduce the mismatch of the DACs in the column driver(s) 104. Such a multi-reference voltage generator (also known as a “reference voltage generator”, a “reference voltage buffer” or a “gamma buffer”) provides low impedance taps in a resistor string of the column drivers 104, and thus make them match better across the display. In addition to matching the LCD column drivers, the reference voltage generator 106 is used to implement gamma correction to improve the contrast of the LCD display, as will now be described.
The data from a video card is usually linear. However, a monitor's output luminance versus input data is nonlinear. Rather, the input data versus output luminance is roughly a 2.2 power function (where L=V^2.2, where L=luminance and V=input data voltage). Accordingly, to display a “correct” luminance, the output should be gamma corrected. This can be accomplished, e.g., by applying the following function to the input data: L′=L^(1/2.5). In addition to correcting the gamma of the LCD display, gamma correction can also stretch the gamma curve to improve the contrast of the display.
Conventionally, LCD monitors have a fixed gamma response. However, LCD manufacturers are beginning to implement dynamic gamma control, where the gamma curve is being updated on a frame-by-frame basis in an attempt to optimize the contrast on a frame-by-frame basis. This is typically accomplished by evaluating the data to be displayed, on a frame-by-frame basis, and automatically adjusting the gamma curve to provide vivid and rich colors.
FIG. 2 shows details of a conventional reference voltage generator 206, which includes an interface control 208, a pair of register banks 210 and 212, multiple (i.e., N) m-bit DACs 220 and multiple (i.e., N) buffers 230.
The interface control 208 may implement an Inter-Integrated Circuit (I2C) bus interface, which is a 2-wire serial interface standard that physically consists of two active wires and a ground connection. The active wires, Serial DAta (SDA) and Serial CLock (SCL), are both bi-directional. The key advantage of this interface is that only two lines (clock and data) are required for full duplexed communication between multiple devices. The interface typically runs at a fairly low speed (100 kHz to 400 kHz), with each integrated circuit on the bus having a unique address.
The interface control 208 receives serial data addressed to the reference voltage generator 206, converts each serial m-bits of display-data into parallel data, and transfers the parallel data bits to the first bank of registers 210. The first bank of registers 210 and the second bank of registers 212 are connected in series, such that once the first bank 210 is full, the data in the first bank 210 can be simultaneously transferred to the second bank 212. Each bank of registers 210 includes, e.g., N separate m-bit registers, where N is the number of multi-level voltage outputs (OUT1-OUTN) produced by the multi-reference voltage generator 206, and m is the number of inputs in each DAC 220.
The two register banks 210 and 212 perform double-buffering to compensate for the slow I2C interface. More specifically, while the data in the N m-bit registers in bank 212 are being converted to analog voltages by the N m-bit DACs, the N m-bit registers in bank 210 are being updated. A problem with this architecture is that for every output, an m-bit DAC 220 is required, thereby impacting the size of the die. If used for dynamic gamma control, each DAC 220 needs time to settle when it is switching between two gamma curves. In most recent applications, dynamic gamma control needs to be switched at line rates and at fast settling times of 500 ns (where the period is approximately 14-20 μs). To handle such switching rates using the architecture in FIG. 2 would require relatively large transistors (which have a relative high cost) and high currents, thereby making it unrealistic for LCD applications where cost and size are of high importance. Additionally, for a same digital code, the output voltages may have large offsets due to mismatches among the multiple DACs 220 and output buffers 230.
Accordingly, it would be beneficial to provide a reference voltage generator that includes less DACs, to thereby reduce the overall die size and cost. It would also be beneficial if such a reference voltage generator can be switched at such a rate that it can be used for dynamic gamma control at line rates. Additionally, it would be beneficial to minimize mismatches that occur within a reference voltage generator.